1. Field of the Invention
The present invention relates an apparatus and a method of manufacturing a laser diode unit. In particular, the present invention relates to an apparatus and a method for mounting a laser diode on a submount.
2. Description of the Related Art
Along with the advancement of high recording density of the hard disk drive (HDD), further improvement is demanded in the performance of a thin film magnetic head. As the thin film magnetic head, a composite type thin film magnetic head is widely used having a structure in which a reproducing head having a magnetoresistive effect element (MR element) for reading and a recording head having an induction-type electromagnetic transducer element for writing are laminated.
A magnetic recording medium used in magnetic recording is made of a discontinuous medium composed of magnetic nanoparticles, and each of the magnetic nanoparticles has a single magnetic domain structure. A recording region (each one bit) is formed by a plurality of the magnetic nanoparticles. Therefore, the boundary of the recording region has an uneven shape. In order to increase the recording density, it is necessary to decrease the unevenness at the boundaries of the recording regions. To that end, it is effective to reduce the size of the magnetic nanoparticles. However, when the magnetic nanoparticles are decreased in size, thermal stability deteriorates due to decrease in volume of the magnetic nanoparticles. In order to enhance the thermal stability, it is preferable that a magnetic material having a large magnetic anisotropy constant KU be used. However, an increase in the anisotropy energy of the magnetic nanoparticles will lead to an increase in the coercive force of the magnetic recording medium so that recording of information using a conventional magnetic head becomes difficult. For this reason, a method is proposed in which, during recording, heat is also applied at the same time when a magnetic field is applied, to lower the coercive force to perform recording. Such a method is referred to as thermally assisted magnetic recording.
In the thermally assisted magnetic recording technology, generating a fine light spot is important, but where and how to provide a light source (laser diode unit) is also a very important problem. In the specification of U.S. Patent Application Publication No. 2008/0043360, a head structure is disclosed in which a laser diode unit having a laser diode mounted therein is mounted on a back surface (back surface of an air bearing surface) of a magnetic head slider on which a recording head and a reproducing head are mounted. This structure is referred to as a composite magnetic head slider structure.
The composite magnetic head slider structure has, in principle, a merit that both the laser diode and the magnetic head slider can be individually assessed in characterization. In a case where the laser diode and the magnetic head slider are prepared on a single wafer in a wafer process, when a defect is found in any one of the laser diode and the magnetic head slider, the magnetic head has to be defective, so that it is possible that the yield of the magnetic head degrades as compared to a conventional magnetic head having only a magnetic head slider. In contrast, in the composite magnetic head slider structure, the laser diode can be independently tested before the laser diode is mounted on the magnetic head slider. Therefore, a defective laser diode can be eliminated in advance, and only a non-defective laser diode is mounted on a magnetic head slider. Therefore, a yield comparable to that of the conventional magnetic head can be ensured.
A laser diode unit can in general be manufactured by the following steps.
(1) A base substrate (submount) is prepared having a mounting surface on which a laser diode is to be mounted. A solder member such as AuSn or the like is formed on the mounting surface. Next, the laser diode is placed on the solder member in such a manner that a metal film such as Au or the like forming an electrode of the laser diode is in contact with the solder member. Thereafter, a pressing load of from several grams to several tens of grams is applied to the laser diode by using a nozzle (which is also referred to as a collet) to press the laser diode against the solder member.
(2) The submount is heated to melt the solder member on the submount.
(3) In order to allow the molten solder member to fuse with the metal film, the heating state is maintained for a certain period of time. Thereafter, the temperature is lowered.
(4) The nozzle is taken away from the laser diode and the pressing load is released.
A problem of the case where such a common manufacturing method is adopted is that a residual stress is generated in the solder member and the laser diode in the steps from (3) to (4).
That is, a stress applied to the solder member and the laser diode by the pressing load of the nozzle remains in the solder member and the laser diode in the cooling and solidifying process of the solder member. A HDD, with which a thermally-assisted head may be used, requires a mean time to failure (MTTF) that is one order of magnitude or more greater than an optical disk drive such as a DVD. Therefore, when a conventional process that has been used for such a DVD or the like is applied to a laser diode for the thermally-assisted head, the subtle stress state becomes a problem.
The residual stress can be released, after the completion of the step (4), by re-melting the solder member in a state in which the pressing load is not applied. This is because the pressing load is not applied so that the solder member itself deforms in the process from re-melting to re-solidifying and the residual stress of solder member is relaxed. However, due to the deformation of the solder member, it is possible that the laser diode that was fixed by the solder member moves relative to the submount. When the laser diode unit, a laser light emitting portion thereof in particular, moves away from an end portion of a waveguide of the magnetic head slider, the laser light cannot be efficiently incident into the waveguide.
A purpose of the present invention is to provide an apparatus and a method of manufacturing a laser diode unit that can reduce the residual stress generated in the solder member and the laser diode when the laser diode is bonded to the submount, and can mount the laser diode on the submount with high positional accuracy.